/Engine/Source/Runtime/CoreUObject/Public/UObject/PackageResourceManager.h
If unsupported, functions without PackageNameOnly in their name that take a PackagePath will behave as if the package does not exist in the repository for unmounted PackageNames If unsupported, functions with PackageNameOnly in their name will indicate the lack of support when called with unmounted PackageNames; see each function's description for its precise behavior
bool SupportsPackageOnlyPaths()